A polymer electrolyte fuel cell in which a hydrogen-containing fuel gas and oxygen-containing oxidizing gas are supplied to an anode and cathode, respectively, and an electromotive force is generated by an electrochemical reaction occurring at both poles is generally constituted of laminating a bipolar plate, a gas diffusion electrode substrate, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode substrate and a bipolar plate in this order. The gas diffusion electrode substrate is required to have high gas diffusivity to allow a gas supplied from the bipolar plate to be diffused into a catalyst layer and high water removal performance to discharge water generated by the electrochemical reaction to the bipolar plate, as well as high electrical conductivity to extract generated electric current. Thus, gas diffusion electrode substrates are widely used in which a microporous layer is formed on a surface of a substrate that is a carbon sheet composed of a carbon fiber or the like.
However, as problems of such gas diffusion electrode substrates, the following problems are known: (1) when the polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of the gas diffusion electrode substrate by liquid water generated in a large amount and shortage in the gas supply, the fuel cell performance is impaired (this problem is hereinafter referred to as “flooding”); and (2) at the time when the gas diffusion electrode substrate is pressed against a bipolar plate in assembly of the polymer electrolyte fuel cell, the gas diffusion electrode substrate bends into a flow channel of a bipolar plate which is engaged in supply of a gas and discharge of generated water so that supply of a gas is hindered, and discharge of generated water from a gas flow channel. Thus, the gas diffusion electrode substrate is required to have mechanical strength required for a gas diffusion electrode substrate while exhibiting high generated water removal performance, and low-bending property such that the gas diffusion electrode substrate is not deformed to a gas flow channel when locally compressed.
For example, there has been proposed a carbon sheet in which to reduce compressive residual strain of a surface of the carbon sheet to suppress bending into a channel provided on a bipolar plate, layers having different bulk densities are laminated in a through-plane direction so that the porous structures of both surfaces each have a density higher than the bulk density of the porous structure of the inner part (Japanese Patent Laid-open Publication No. 2007-176750).
There has been proposed a fuel cell gas diffusion substrate in which the resin density of a carbon sheet is continuously decreased in a through-plane direction to prevent deterioration of performance of the fuel cell (Japanese Patent Laid-open Publication No. 2013-145640).
However, Japanese Patent Laid-open Publication No. 2007-176750 still has the problem that since the bulk densities of both surfaces of the carbon sheet are equivalent to each other, water, when generated, is easily retained in the inner part so that marked flooding occurs.
Japanese Patent Laid-open Publication No. 2013-145640 still has the problem that since the amount of a carbonized resin on the bipolar plate side decreases, not only bending of a carbon sheet into a bipolar plate flow channel increases, but also binding of a carbon fiber on a surface is loosened so that the carbon fiber fluffs in the bipolar plate, intercepts generated water in the flow channel, and blocks the flow channel.
Thus, it could be helpful to provide a carbon sheet suitable for use in a gas diffusion electrode substrate having an excellent anti-flooding characteristic that has been heretofore difficult to achieve, and that can be prevented from bending into a gas flow channel.
It could also be helpful to provide a gas diffusion electrode substrate obtained using the carbon sheet as a substrate, and a fuel cell including the gas diffusion electrode.